The present invention is directed to a method and apparatus for quality monitoring of at least two series connected transmission sections of a digital signal transmission link for transmission equipment conforming to a synchronous hierarchy, whereby digital signals are transmitted in virtual containers of synchronous transport modules and section overheads of the synchronous transport of the transmission link having parity bytes and a special byte reserved for special use. The parity bytes are derived from the received digital signals at the respective end of the transmission sections and are compared to the received parity bytes and an error signal is formed in a prescribed dependency on the result of this comparison.
A method of the type initially cited is known from CCITT-recommendations G707 through G709 (hereby incorporated by reference) and from the German periodical NTZ, Vol. 41 (1988) No. 10, Pages 570-574. In the prior art method, bytestructured PCM signals are transmitted together with a parity byte. The parity bytes serve the purpose of calculating the bit error rate during the transmission. In the control case, the parity is formed from the bits of an entire frame or of a sub-frame and these bits are then transmitted in the following frame or sub-frame. The parity byte B1 is checked by every intermediate regenerator and is reformed. Errors in a transmission link can thereby be restricted to the affected regenerator section. By contrast, the parity byte B2 serves the purpose of bit error acquisition of a transmission link between two line ends. The parity byte B3 is provided in the path overhead POH of each and every virtual container and is therefore not available for a quality monitoring of parts of the physical network.
Since the parity byte B1 is respectively reformed at the beginning of the regenerator section and the parity byte B2 is respectively reformed at the start of the transmission link, the regenerator sections or transmission links can be individually monitored. As a result errors in a transmission link can be limited to the affected regenerator section or errors on a transmission path can be limited to the affected transmission link.
Furthermore, Telcom report, special, Vol. 10, December 1987, Multiplexing Line Transmission, Pages 107-112 discloses that digital transmission systems be provided with a means for on-line monitoring. In these systems the intermediate regenerators thereby each contain a monitoring circuit that monitors transgressions of the code rule via the ongoing digital sum. Apart from error bursts, conclusions about the bit error frequency can thus be made at the appertaining location. What is referred to as the error rate is transmitted to the locating apparatus in coded form. The bit error rate at each and every individual intermediate regenerator is thus acquired in the locating apparatus. Conclusions about the transmission quality of the individual regenerator fields can be made in this manner in a monitoring terminal station by a comparison of the measured bit error rates, so that an individual monitoring of transmission sections is also possible.
According to the CCITT recommendations G707 . . . 709, particularly pages 83-84, Chapter 5.21 and Page 93, FIG. 5.1, the parity bytes B1 and B2 are available for quality monitoring in the section overhead SOH, namely one parity byte B1 per synchronous transport module STM-N and one parity byte B2 in every synchronous transport module STM-1. The parity byte B1 allows the monitoring of an individual regenerator section and the parity byte B2 allows the monitoring of a digital signal line section. A through monitoring across a plurality of digital signal line sections is not provided with these two parity bytes according to the CCITT recommendations, since it is not possible for the section overhead SOH to balance frequency differences upon transition from digital signal line section to digital signal line section. It is therefore not possible to produce a fixed linkage of parity bytes in the section overhead SOH to the useful information in the virtual container from the input up to the output of a digital signal line.